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The C Standard, 6.7.3.2, paragraph 20 [ISO/IEC 9899:2024], says

As a special case,

...

the last element of a structure with more than one named member

...

may have an incomplete array type;

...

this is called a flexible array member. In most situations, the flexible array member is ignored. In particular, the size of the structure is as if the flexible array

...

This technique has widely been known as the "struct hack." However, the classical method of the "struct hack" results in undefined behavior, although it may work most of the time.

Non-Compliant Code Example

The following is the old, incorrect "struct hack" which results in undefined behavior.

member were omitted except that it may have more trailing padding than the omission would imply.

The following is an example of a structure that contains a flexible array member:

struct flex_array_struct {
  int num;
  int data[];
};

This definition means that when computing the size of such a structure, only the first member, num, is considered. Unless the appropriate size of the flexible array member has been explicitly added when allocating storage for an object of the struct, the result of accessing the member data of a variable of nonpointer type struct flex_array_struct is undefined. DCL38-C. Use the correct syntax when declaring a flexible array member describes the correct way to declare a struct with a flexible array member.

To avoid the potential for undefined behavior, structures that contain a flexible array member should always be allocated dynamically. Flexible array structures must

• Have dynamic storage duration (be allocated via malloc() or another dynamic allocation function)
• Be dynamically copied using memcpy() or a similar function and not by assignment
• When used as an argument to a function, be passed by pointer and not copied by value

Noncompliant Code Example (Storage Duration)

This noncompliant code example uses automatic storage for a structure containing a flexible array member:

#include <stddef.h>
 
struct flex_array_struct {
  size_t num;
  int data[];
};
 
void func(void) {
  struct flex_array_struct flex_struct;
  size_t array_size = 4;

  /* Initialize structure */
  flex_struct.num = array_size;

  for (size_t i = 0; i < array_size; ++i) {
    flex_struct.data[i] = 0;
  }
}

Because the memory for flex_struct is reserved on the stack, no space is reserved for the data member. Accessing the data member is undefined behavior.

Compliant Solution (Storage Duration)

This compliant solution dynamically allocates storage for flex_array_struct:

#include <stdlib.h>
 
struct flex_array_struct {
  size_t num;
  int data[];
};
 
void func(void) {
  struct flex_array_struct *flex_struct;
  size_t array_size = 4;

  /* Dynamically allocate memory

struct hack {
     int num;
     char my_char;
     int data[1];
};

...
/* Space is allocated for the struct */
  flex_struct hack= *hackP = malloc((struct flex_array_struct *)malloc(
    sizeof(struct hack) flex_array_struct)
    + sizeof(int) * (ARRAY_SIZE-1)array_size);
  if (!hackPflex_struct == NULL) {
     /* handleHandle malloc failureerror */
  }
hackP->num = SOME_NUMBER;
hackP->my_char = SOME_CHAR;

/* AccessInitialize data[] as if it had been allocated as data[ARRAY_SIZE] */
for (structure */
  flex_struct->num = array_size;

  for (size_t i = 0; i < ARRAYarray_SIZEsize; i++i) {
    hackPflex_struct->data[i] = i0;
  }

This is, strictly speaking, undefined behavior when accessing {{hackP->data\[i\]}} for i > 0, because that goes beyond the bounds of the array.

Compliant Solution

}

Noncompliant Code Example (Copying)

This noncompliant code example attempts to copy an instance of a structure containing a flexible array member (struct flex_array_struct) by assignment:

#include <stddef.h>
 
struct flex_array_struct {
  size_t num;
  int data[];
};
 
void func(struct flex_array_struct *struct_a,
          struct flex_array_struct *struct_b) {
  *struct_b = *struct_a;
}

When the structure is copied, the size of the flexible array member is not considered, and only the first member of the structure, num, is copied, leaving the array contents untouched.

Compliant Solution (Copying)

This compliant solution uses memcpy() to properly copy the content of struct_a into struct_b:The solution is part of the C99 standard, the flexible array member.

#include <string.h>
 
struct flex_array_struct flexArrayStruct{
  intsize_t num;
  char my_char;
  int data[];
};

...
/* Space is allocated for the struct */
struct flexArrayStruct *structP = malloc(sizeof(struct flexArrayStruct) + sizeof(int) * ARRAY_SIZE);
if (!structP) {
     /* handle malloc failure */
}

/* Now, it is as if the struct were defined
	struct {int num; char my_char; int data[ARRAY_SIZE];} *structP;
and we can access the elements as if they were so. */
structP->num = SOME_NUMBER;
structP->my_char = SOME_CHAR;

/* Access data[] as if array had been allocated as data[ARRAY_SIZE] */
for ( 
void func(struct flex_array_struct *struct_a,
          struct flex_array_struct *struct_b) {
  if (struct_a->num > struct_b->num) {
    /* Insufficient space; handle error */
    return;
  }
  memcpy(struct_b, struct_a,
         sizeof(struct flex_array_struct) + (sizeof(int)
           * struct_a->num));
}

Noncompliant Code Example (Function Arguments)

In this noncompliant code example, the flexible array structure is passed by value to a function that prints the array elements:

#include <stdio.h>
#include <stdlib.h>
 
struct flex_array_struct {
  size_t num;
  int data[];
};
 
void print_array(struct flex_array_struct struct_p) {
  puts("Array is: ");
  for (size_t i = 0; i < struct_p.num; ++i) {
    printf("%d ", struct_p.data[i]);
  }
  putchar('\n');
}

void func(void) {
  struct flex_array_struct *struct_p;
  size_t array_size = 4;

  /* Space is allocated for the struct */
  struct_p = (struct flex_array_struct *)malloc(
    sizeof(struct flex_array_struct)
    + sizeof(int) * array_size);
  if (struct_p == NULL) {
    /* Handle error */
  }
  struct_p->num = array_size;

  for (size_t i = 0; i < array_size; ++i) {
    struct_p->data[i] = i;
  }
  print_array(*struct_p);
}

Because the argument is passed by value, the size of the flexible array member is not considered when the structure is copied, and only the first member of the structure, num, is copied.

Compliant Solution (Function Arguments)

In this compliant solution, the structure is passed by reference and not by value:

#include <stdio.h>
#include <stdlib.h>
 
struct flex_array_struct {
  size_t num;
  int data[];
};
 
void print_array(struct flex_array_struct *struct_p) {
  puts("Array is: ");
  for (size_t i = 0; i < ARRAYstruct_SIZEp->num; i++i) {
    printf("%d ", structPstruct_p->data[i] = i;
}

No actual array space is allocated within the struct object. By mallocating space for the struct, the space computation is done at runtime, and allows the programmer to dynamically allocate space for an array within a struct.

However, some restrictions do apply. The incomplete array type MUST be the last element within the struct. You also cannot have an array of objects if the object has flexible array members. Nor can such object be used as a member in the middle of another struct.

Risk Assessment

);
  }
  putchar('\n');
}

void func(void) {
  struct flex_array_struct *struct_p;
  size_t array_size = 4;

  /* Space is allocated for the struct and initialized... */

  print_array(struct_p);
}

Risk Assessment

Failure to use structures with flexible array members correctly can result in undefined behavior. Although the old "stack hack" method results in undefined behavior, it will work most of the time.

References

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Related Guidelines

Key here (explains table format and definitions)

CERT-CWE Mapping Notes

Key here for mapping notes

CWE-401 and MEM33-CPP

There is no longer a C++ rule for MEM33-CPP. (In fact, all C++ rules from 30-50 are gone, because we changed the numbering system to be 50-99 for C++ rules.)

Bibliography

...

Image Added Image Added Image Added Wiki Markup\[[McCluskey 01|AA. C References#McCluskey 01]\] ;login:, July 2001, Volume 26, Number 4 \[[ISO/IEC 9899-1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.7.2.1, "Structure and union specifiers"